A series of short starter bumps with a 12 volt battery won't hurt it, but I'd recommend disconnecting the wire that runs from the big terminal on the starter to the ammeter to protect lights and other 6V components. The starter is rugged enough to handle short bursts of running on 12V as long as you don't run it for more than about 5 seconds at a time, or do it repeatedly. You can even do a compression test that way if you use a little common sense.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!